If you don’t have dogs and you can use a pressure Mat under the mattress. A lot of hospitals and nursing homes use these. They’re pretty easy to connect to SmartThings.
However, it can get tricky if you have big dogs they get on the bed. And even two people in one bed can be complicated as far as where exactly to put the mats.
Samsung announced a sleep sensor last year that would do exactly this, but then they never brought it to market.
There are all kinds of other complex ways, but it just depends on exactly what you want to set up.
A pressure mat is the most straightforward for “someone is in this bed” Everything else relies on assumptions.